Method for 2D/3D inspection of an object such as a wafer

ABSTRACT

A method is provided for inspecting the surface of an object such as a wafer having tridimensional structures, using a confocal chromatic device with a plurality of optical measurement channels and a chromatic lens allowing optical wavelengths of a broadband light source to be focused at different axial distances defining a chromatic measurement range. The method includes a step of obtaining an intensity information corresponding to the intensity of the light actually focused on an interface of the object within the chromatic measurement range at a plurality of measurement points on the object by measuring a total intensity over the full spectrum of the light collected by at least some of the optical measurement channels in a confocal configuration.

BACKGROUND

The invention relates to a method for inspecting an object such as awafer, and more particularly for inspecting an object comprisingstructures such as bumps or micro-bumps.

The field of the invention is, but not limited to, 2D-3D inspection andmetrology for semiconductor industry.

Tridimensional structures such as bumps, micro-bumps, solder bumps,copper pilar, copper nails, Re-Distribution Layers (RDL), Under Bumpmetallization (UBM), metal patterns are more and more widely used forinterconnections and other application in the semiconductor industry.With the evolution of the IC packaging, the critical dimensions of thesestructures tend to decrease and their density on wafers or chips tend toincrease. In the same time there is a need for inspection systemscapable of inspecting all of these structures at high speed duringproduction, as a few defects are sufficient to render corresponding ICsdefective.

Chromatic confocal technique is a well-known technique fortridimensional (3D) surface mapping or profilometry, in particular forsemiconductor applications.

The technique relies on the use of a chromatic lens with an enhancedchromatism, whose focal length depends strongly on the opticalwavelength. Each wavelength of the light crossing such lens is focusedat a different distance, or in a different focal plane.

The chromatic lens is embedded in a confocal set-up with source anddetection pinholes (usually made by optical fibers core) placed at theconfocal planes of the chromatic lens to reject out-of-focus lens. Whena reflecting surface is placed in front of the chromatic lens, only thelight with the wavelength whose focal plane corresponds to the positionof the surface is transmitted by the detection pinhole.

Detection is made by a spectrometer, which comprises usually adispersing element and a linear or matrix sensor (CCD or CMOS) toacquire the intensity spectrum of the light. The height (or distance) ofthe surface relative to the chromatic lens is obtained by analyzing theintensity spectrum of the detected light.

Such set-up allows measuring distances on a single point at the time. Soinspecting a full wafer surface by scanning all the surface may be verytime-consuming. Actually, the factor limiting the measurement speed isthe readout time of the linear sensor for acquiring the intensityspectrum.

Acquisition speed can be improved by providing several measurementchannels in parallel.

We know for instance the document US 2015/0260504 which discloses animplementation of a confocal chromatic device in which severalmeasurement channels are provided through a chromatic lens with severaloptical fibers. The sensor allows measuring distance or height atseveral points on the surface of the object simultaneously.

However, even if the acquisition rate is improved, the time forinspecting a full wafer surface remains very long.

Another issue when measuring or inspecting patterned wafers is theaccurate localization of the measurement points relative to the existingstructures. That issue is usually solved by using a 2D (bidimensional)inspection or imaging system such as a camera.

We know for instance the document U.S. Pat. No. 6,934,019 whichdescribes an inspection system based on a chromatic confocal sensorwhich comprises also an imaging camera. The measurements require twosteps: first acquiring an image of the wafer with the camera andcomputing a map of the locations of the structures to be measured; andsecond performing the height measurements.

However, the switching between the camera and the chromatic sensor istime consuming and the need for mechanical displacements to positioneither the camera or the chromatic confocal sensor above the structuresto be measured may impact the positioning accuracy for the heightmeasurement.

It is an object of the invention to provide a method allowing fast andaccurate 2D (bidimensional or in-plane imaging) inspection of an objectsuch as a wafer.

It is also an object of the invention to provide a method allowing fastand accurate 3D (tridimensional height measurements) inspection of anobject such as a wafer.

It is also an object of the invention to provide a method allowingproviding simultaneously or at least during a same scan and withminimized positioning uncertainty an intensity image (2D) and accurateheight measurements (3D) on an object such as a wafer with patterns orstructures.

It is also an object of the invention to provide a method allowingaccurate locating of height measurement positions, and/or accuratepositioning of height measurement probes relative to structures orpatterns on an object such as a wafer.

It is also an object of the invention to provide a method allowingcharacterizing or inspecting structures (in 2D and/or in 3D) of anobject such as a wafer in position and shape.

It is also an object of the invention to provide a method for inspectingbumps, trenches and other patterned structures on an object such as awafer.

SUMMARY

Such objects are accomplished through a method for inspecting thesurface of an object such as a wafer comprising tridimensionalstructures, using a confocal chromatic device with a plurality ofoptical measurement channels and a chromatic lens allowing opticalwavelengths of a broadband light source to be focused at different axialdistances defining a chromatic measurement range, characterized in thatit comprises a step of obtaining an intensity information correspondingto the intensity of the light actually focused on an interface of theobject within the chromatic measurement range at a plurality ofmeasurement points on the object by measuring a total intensity over thefull spectrum of the light collected by at least some of the opticalmeasurement channels in a confocal configuration.

The intensity information may thus correspond to pixel elements of animage of the object acquired with a depth of focus extending over thechromatic measurement range, thus well beyond the depth of focusachieved for a single wavelength.

According to some modes of realization, the method may comprise:

-   -   providing a chromatic lens with an extended axial chromatism;    -   illuminating the object through the chromatic lens with a        plurality of optical wavelengths being focused at different        axial distances;    -   collecting the light reflected by the object through the        chromatic lens at a plurality of measurement points using a        plurality of optical measurement channels with collection        apertures;    -   measuring a total intensity of the light collected by at least        one of the optical measurement channels for obtaining an        intensity information.

According to some modes of implementation, the method of the inventionmay further comprise a step of measuring a spectral information of thelight collected by an optical measurement channel for obtaining an axialdistance information within the chromatic measurement range.

According to some modes of implementation, the method of the inventionmay further comprise a step of locating a structure on the surface ofthe object using intensity information.

According to some modes of implementation, the method of the inventionmay further comprise steps of:

-   -   identifying a measurement point of interest using intensity        information;    -   obtaining an axial distance information at said point of        interest.

According to some modes of implementation, the method of the inventionmay further comprise at least one of the following steps:

-   -   identifying a measurement point of interest relative to a        structure;    -   identifying a measurement point of interest relative to a        structure corresponding to a summit of said structure.

The method of the invention may further comprise a step of deducing aheight information of said structure.

According to some modes of implementation, the method of the inventionmay further comprise a step of moving relatively the object and thechromatic lens to position an optical measurement channel so as toobtain an axial distance information on a previously identifiedmeasurement point of interest.

According to some modes of implementation, the method of the inventionmay further comprise a step of moving relatively the object and thechromatic lens along a pre-defined scan trajectory, and for a scanposition:

-   -   obtaining an intensity information; and/or    -   obtaining an axial distance information on a measurement point        of interest previously identified.

The movement along the scan trajectory may be done continuously, withthe measurement being acquired “on-the-flight” at the scan positions.The movement along the scan trajectory may also be done step-by-step,with a stop at the scan positions.

The intensity information provides elements or pixels of a 2Drepresentation corresponding to an image of the surface of the object.As it will be explained later, it may be obtained at a high acquisitionrate with a high resolution thanks to the extended depth of focus of theacquisition device. The extended depth of focus allows in particular toimage in good quality structures or patterns of the surface of theobject extending in altitude or depth.

Such intensity image may be used for locating specific patterns orstructures on the surface of the object, and thus locating accuratelymeasurement points of interest where a depth or distance informationshall be measured. So, the depth measurements which are much slower aredone only at the relevant points of interest.

Of course, the intensity information and the axial distance/depthinformation may also be acquired in a systematic manner on a same or adifferent sampling pattern (corresponding for instance to a samplinggrid) of measurement points at the surface of the object, so as toprovide a 2D intensity image and a 3D altitude map of that surface. Inparticular:

-   -   The intensity information and the axial distance/depth        information may be acquired according to different sampling        patterns so as to provide a 3D altitude map with a spatial        resolution coarser than the spatial resolution of the 2D        intensity image;    -   The axial distance/depth information may be acquired according        to a sampling pattern which is a subset of the sampling pattern        used for acquiring the intensity information.

According to some modes of implementation, the method of the inventionmay comprise at least one of the following steps:

-   -   changing a spacing of measurement points;    -   changing a spacing of measurement points by changing a scaling        factor between a spatial repartition of collection apertures of        the optical measurement channels and the measurement points        using a magnifying lens.

The spacing of measurement points may thus be adjusted by a magnifyinglens or any other means, such as for instance by physically movingcollection apertures of optical measurement channels relative to thechromatic lens.

According to some modes of implementation, the method of the inventionmay further comprise at least one of the following steps:

-   -   adjusting a spacing of measurement points taking into account a        spatial repartition of structures on the object;    -   adjusting a spacing of measurement points so as to substantially        match a spacing of structures on the object.

The spacing of the structures may for instance be a center-to-centerspacing, or a side-to side spacing.

The method of the invention may further comprise a step of:

-   -   obtaining an information on the spacing of structures using        a-priori knowledge on the object (or a description information        of the object);    -   obtaining an information on the spacing of structures using        intensity information and/or axial distance information        previously obtained.

According to some modes of implementation, the method of the inventionmay further comprise steps of:

obtaining an intensity information and/or an axial distance informationat a plurality of measurement points with a first spacing of saidmeasurement points, for locating sub-elements on the surface of theobject;

-   -   obtaining an intensity information and/or an axial distance        information at a plurality of measurement points with a second        spacing of said measurement points finer than the first spacing        on a sub-element.

According to some modes of implementation, the method of the inventionmay further comprise steps of positioning relatively the object and thechromatic lens along an axial direction using an intensity informationand/or an axial distance information.

Such positioning along the axial direction may be done for positioningthe surface of the object within the measurement range of the chromaticlens. It may be done:

-   -   Using an axial distance information which provides directly a        distance measurement or an out-of-range information (no        measurements);    -   Using an intensity information. That solution has the advantage        of allowing a high acquisition speed for positioning the object        within the measurement range of the chromatic lens. Due to the        confocal characteristic of the optical arrangement, a        significant intensity is measured only when an interface of the        object is present within the measurement range. Of course, once        positioned in range, an axial distance information may be used        for a fine adjustment.

According to some modes of implementation, the method of the inventionmay further comprise steps of:

-   -   building an intensity image by combining intensity information        obtained in a region of interest of the object; and/or    -   building a height map by combining axial distance information        obtained in a region of interest of the object.

According to some modes of implementation, the method of the inventionmay further comprise a step of comparing obtained axial distanceinformation with reference value(s).

According to some modes of implementation, the method of the inventionmay be implemented for inspecting tridimensional structures of at leastone of the following type: bumps, micro-bumps, solder bumps, copperpilar, copper nails, Re-Distribution Layers (RDL), metal patterns.

The method of the invention may be carried out with any chromaticconfocal device with compatible features.

The method of the invention may also be carried out with a confocalchromatic device for inspecting the surface of an object such as awafer, comprising:

-   -   a chromatic lens with an extended axial chromatism;    -   a light source for illuminating the object through the chromatic        lens with a plurality of optical wavelengths being focused at        different axial distances;    -   a plurality of optical measurement channels with collection        apertures arranged for collecting the light reflected by the        object through the chromatic lens at a plurality of measurement        points;

wherein the plurality of optical measurement channels comprises opticalmeasurement channels with an intensity detector for measuring a totalintensity of the collected light.

The chromatic lens may comprise any kind of chromatic lens or lensassembly having a suitable chromatic aberration over a field of view,such as for instance:

-   -   a single lens or lens assembly shared between the optical        measurement channels;    -   a plurality of lenses or microlenses each used by only one or        several optical measurement channels;    -   holographic elements;    -   diffractive lens or microlens elements.

The chromatic lens may comprise at least a lens made with a dispersivematerial, and any other lenses required for providing the necessaryoptical arrangement. Such lens may be designed according to well-knowntechniques so as to provide a strong chromatic aberration, allowingdifferent optical wavelengths crossing the lens to be focused atdifferent distances, and that over a lateral field of view.

The confocal chromatic device of the invention thus comprises severaloptical measurement channels. Each optical measurement channel issensitive to the light reflected at a specific measurement point in aplane perpendicular to the optical axis of the chromatic lens, and alonga range of axial distances or heights (in a direction substantiallyparallel to the optical axis of the lens) corresponding to the planes offocalization of the different optical wavelengths crossing the chromaticlens. That range of axial distances allowing measurement may be definedas the measurement range of the device.

In other words, the measurement points correspond to the conjugatepoints of the collection apertures, or, more precisely, to theprojection of the conjugate points of the collection apertures for thedifferent wavelengths on a plane perpendicular to the optical axis ofthe chromatic lens. These collection apertures operate as pinholesallowing rejecting out-of-focus light, according to a classical confocaldetection scheme.

The light source may comprise any kind of light source capable ofemitting light at a plurality of wavelengths covering a spectral rangefor which the chromatism of the chromatic lens is efficiently usable. Itmay comprise for instance light-emitting diodes (LED), thermal lightsources such as halogen lamps, or gas-discharge lamps. It may alsocomprise a tunable laser, a white laser or a supercontinuum photonicsource. The light source may generate light with wavelengths within forinstance a range of 400-700 nm (visible range) allowing inspection ofsurfaces and/or transparent layers in the visible range. Alternatively,the light source may generate light with wavelengths above 1 micron inthe infrared range, allowing for instance inspections through layers ofsilicon or other materials transparent in the infrared.

The light source may comprise a single light source shared between allthe optical measurement channels, or a plurality of light sources eachshared between several optical measurement channels, or a light sourceper optical measurement channel.

Intensity detectors may comprise any photodetector measuring anintensity of light, or a global intensity of light over a spectralrange.

According to some modes of realization, intensity detectors maycomprise:

-   -   Separate or discrete intensity detectors for each optical        measurement channel, such as for instance phototransistors,        photodiodes or avalanche photodiodes; and/or    -   Intensity detectors shared between a pluralities of optical        measurement channel. Such intensity detectors may comprise for        instance photodiode arrays, line or matrix CCD or CMOS in which        intensity measurements of different optical measurement channels        are done on different pixels.

The intensity detectors provide a global intensity of light at themeasurement point. So they provide a 2D image information of the object.

The 2D measurements benefit from an extended depth of focus, because ofthe chromatic confocal set-up. The image which is obtained by thesemeans is in focus or well-focused over the whole measurement range ofthe device, because it is done mostly using the wavelength focused onthe surface of the object, whatever position that surface may have inthe measurement range. So the available depth of focus for the imagingis determined by the extent of the chromatic aberration of the chromaticlens. It is thus much larger than the depth of focus which would beavailable with a classical achromatic lens, and which correspond to thedepth of focus available for a single wavelength with the chromaticlens.

According to some modes of realization of the invention, the pluralityoptical of measurement channels may further comprise at least oneoptical measurement channel with a spectral detector for measuring aspectral information of the collected light and deducing an axialdistance information.

Such spectral detector(s) may comprise any detector capable of providingan information relative to an intensity of light in function of opticalwavelengths, such as for instance:

-   -   Spectrometer type devices with a dispersing element such as a        grating or a diffraction array and a sensor capable of        collecting a light intensity for the different wavelengths, such        as for instance a line CCD, CMOS or a photodiode array;    -   Devices with color filters in front of a line or matrix        detector, allowing a detection selective in wavelength with        different detector areas.

Spectral detector may also comprise detectors shared between severaloptical measurement channels, such as line or matrix CCD or CMOS. Inthat case, intensity spectra of different optical measurement channelsare collected on different areas or pixels of the detector.

The axial distance information may be deduced from the intensityspectrum by identifying the peak(s) in the spectrum or the wavelengthswhich are the most reflected, and which are representative of thelocation of the corresponding interfaces of an object in the measurementrange. Of course, in presence of a transparent object with severaldetectable layers, several peaks representative of optical distances toseveral interfaces may be identified.

So, the spectral detectors provide an axial distance, or a heightinformation at the measurement point. They thus provide a 3D informationwhich is the usual purpose of the chromatic confocal sensors.

The invention thus allows doing a sensor with 2D-3D inspectioncapabilities in a single measurement head. The measurement points forthe 2D and 3D inspection are in a fixed, stable and well known spatialrelationship.

2D total intensity measurement can be done much faster than 3D axialdistance measurements, because their only limitation in terms ofacquisition rate relate to the integration time or bandwidth of thedetector. In the other hand, 3D axial measurement rates are limited atleast by the integration time and readout time of spectrometer sensors.As consequence, 2D measurement may be done at acquisitions rates 10times or even much faster than 3D measurements. For instance, 2Dmeasurement may be done at acquisition rates of several tens ofkilohertz (for instance 50 KHz to 100 KHz), whereas 3D measurements maybe done only at acquisition rates of a few kilohertz.

So, the device of the invention is particularly well adapted for highspeed inspection, because it allows for instance:

-   -   fast 2D inspection with an extended depth of focus, allowing for        instance inspection of the surface of an object with extended        tridimensional structures (such as bumps, pillars, nails, . . .        ) with an optimal lateral resolution at any measurement points        without refocusing; and/or    -   fast 2D inspection of the surface of a structured object, and        on-the-flight 3D measurement at selected points of interest.

According to some modes of realization, the optical measurement channelsmay comprise optical waveguides, or planar optical waveguides.

According to some modes of realization, the optical measurement channelsmay comprise optical fibers.

According to some modes of realization, the broadband light source maybe conveyed through illumination apertures arranged in a confocalconfiguration relative to the chromatic lens and the collectionapertures.

The device of the invention may then comprise a beam splitter insertedbetween the chromatic lens and, respectively, the illumination aperturesand the collection apertures. The beam splitter may be preferablyinserted in a part where the propagating beams are collimated, forinstance using collimating lenses. Or course, the collection aperturesand the illumination apertures shall be arranged so that a collectionaperture and an illumination aperture are both conjugate points of asame measurement point, through the beam splitter and the chromaticlens.

According to some modes of realization, the optical measurement channelsmay comprise illumination optical fibers, an end of which being used asillumination apertures.

These illumination optical fibers may comprise multimode, or single modefibers. They may be arranged or grouped in bundles. They may have an endcorresponding to the illumination apertures positioned in a mount piecewith for instance v-grooves for accurate positioning.

According to some modes of realization, the device of the invention maycomprise collection apertures comprising, for instance:

-   -   pinholes, like through holes in a mask or a wall;    -   an entrance slit, corresponding to a through aperture elongated        on one direction, which materializes several collection        apertures for several optical measurement channels arranged in        line;    -   pixels or detection elements of a detector.

According to some modes of realization, the optical measurement channelsmay comprise collection optical fibers, an end of which being used ascollection apertures.

These collection fibers may comprise multimode, or single mode fibers.They may be arranged or grouped in bundles. They may have an endcorresponding to the collection apertures positioned in a mount piecewith for instance v-grooves for accurate positioning.

According to some modes of realization, the broadband light source maybe conveyed by the collection optical fibers.

The optical measurement channels may then comprise a coupler or a fibercoupler for directing the light of the light source to the collectionaperture, and directing the light collected back by the collectionaperture towards a detector.

According to some modes of realization, the device of the invention maycomprise at least one optical routing element allowing doing at leastone of the following:

-   -   Using an intensity detector and a spectral detector to do        measurements simultaneously or sequentially on one optical        measurement channel;    -   Selectively using an intensity detector and/or a spectral        detector with a plurality of optical measurement channels.

Such optical routing element may comprise for instance:

-   -   A coupler for splitting the light collected on an optical        measurement channel between an intensity detector and a spectral        detector. In that case, total intensity and spectral information        may be obtained on a same optical measurement channel in        parallel (in a synchronous or asynchronous way), or        sequentially;    -   An optical switch for directing the light collected on an        optical measurement channel towards either an intensity detector        or a spectral detector. In that case, total intensity and        spectral information may be obtained on a same optical        measurement channel sequentially;    -   An optical multiplexer, comprising for instance several optical        switches, for selectively connecting intensity detector(s)        and/or spectral detector(s) to several measurement channels, so        as for instance to share such intensity detector(s) and/or        spectral detector(s) between several optical measurement        channels, or to select intensity detector(s) and/or spectral        detector(s) to connect to one optical measurement channel.

According to some modes of realization, the device of the invention mayfurther comprise a magnifying lens positioned between the collectionapertures and the chromatic lens, and arranged for introducing avariable or changeable scaling factor between the spatial repartition ofthe collection apertures and the measurement points.

The magnifying lens may comprise any kind of lens or lens assembly. Itmay be essentially achromatic for the used wavelengths (or achromaticfor the wavelengths of the light source used in detectors).

The device may of course comprise other lenses placed between thecollection apertures and the magnifying lens, and/or other lenses placedbetween the magnifying lens and the chromatic lens.

Several configurations are possible.

According to some modes of realization, the device of the invention maycomprise a magnifying lens and a chromatic lens arranged so that toprovide an intermediate conjugate focal plane which is simultaneously:

-   -   A conjugate focal plane of the plane with the collection        apertures for a lens assembly comprising the magnifying lens;        and    -   A conjugate focal plane of the plane with the measurement points        for a lens assembly comprising the chromatic lens.

According to some modes of realization, the intermediate conjugate focalplane may be:

-   -   At a finite distance; or    -   At a finite distance and positioned between the magnifying lens        and the chromatic lens.

It may then form a real image plane of the collection apertures.

According to some modes of realization, the intermediate conjugate focalplane may be at infinite distance, corresponding to collimated beams.

According to some modes of realization, the device of the invention maycomprise a collimating lens positioned between the collection aperturesand the magnifying lens.

Such collimating lens may for instance be uses with a beam splitter aspreviously explained. It may be arranged with the collection aperturespositioned in its focal plane, so as to provide to the magnifying lenscollimated beams.

The device of the invention may then comprise a magnifying lens with anafocal lens arrangement.

Such afocal lens arrangement has an infinite effective focal length. Itmay be done for instance with two converging lenses positioned so thattheir spacing corresponds to the sum of their focal lengths (or theirintermediate focal planes are at the same position).

In all cases, the plane with the measurement points is an image plane(or a conjugate plane) of the plane with the collection apertures by thewhole optical assembly comprising the magnifying lens and the chromaticlens. Or in other words the measurement points are respective images ofthe collection apertures by that whole optical assembly. Such images areformed with a lateral magnification factor which depend on themagnification lens (for a given or particular chromatic lens of course).So, changing the magnification factor provided by the magnifying lensallows changing the spatial repartition of the measurement points by ascaling factor without changing the chromatic lens.

Of course, the scaling factor or the magnification factor may correspondto a magnification (absolute value higher than one), a reduction(absolute value lower than one) or a unity magnification (absolute valueequal to 1).

The use of the magnifying lens allows changing the spatial repartitionof the measurement points, continuously and/or by discrete steps,without changing the chromatic lens and thus without changingsignificantly the measurement range defined by the chromatic dispersionof that chromatic lens.

In addition, by taking care of providing magnifying lenses arrangementswhich allow positioning the intermediate conjugate focal plane at thesame position along the optical axis relative to the chromatic lens (orat infinity), the chromatic lens is always used in similar conditions.

According to some modes of realization, the device of the invention maycomprise a magnifying lens of a zoom type allowing introducing avariable magnification.

The magnifying lens may comprise for instance a zoom lens, or amagnifying lens or lens assembly of a zoom type.

Such magnifying lens (of a zoom type) may comprise:

-   -   At least one lens movable (along the optical axis) and allowing        varying a magnification;    -   A lens arrangement allowing varying the magnification between        the plane of the collection apertures and an intermediate        conjugate focal plane at finite distance;    -   An afocal zoom arrangement allowing modifying the width of        collimated beams, for operating with an intermediate conjugate        focal plane at infinite distance.

The device of the invention may further comprise a mechanical mountallowing changing at least one of the following:

-   -   a magnifying lens,    -   a combination of magnifying lens and chromatic lens.

The mechanical mount may comprise for instance a turret or a linearstage for changing a magnifying lens.

It may allow combining several magnifying lenses with differentmagnification with one chromatic lens. For instance, it may hold severalmagnifying lenses on a moving stage (such as a turret or a linear stage)so as to be able to position any of them between the collectionapertures and the chromatic lens.

Of course, the mechanical mount may allow changing magnifying lenses, atleast some of which being magnifying lens of a zoom type.

As previously said, the device of the invention may comprise amechanical mount allowing changing a combination of magnifying lens andchromatic lens, such as:

-   -   Several magnifying lenses with several chromatic lens;    -   One magnifying lens with several chromatic lenses.

According to some modes of realization, the device of the invention mayfurther comprise collection apertures respectively arranged along afirst line and a second line substantially parallel to the first line,the first line comprising collection apertures of optical measurementchannels with an intensity detector, the second line comprisingcollection apertures of optical measurement channels with a spectraldetector.

Such configuration allows for instance acquiring 3D spectral informationwith optical measurement channels of the second line on measurementpoints of interest selected using 2D total intensity informationacquitted at higher rate using optical measurement channels of the firstline, during a monotonous relative displacement of the object relativeto the chromatic lens.

According to some modes of realization, the device of the invention mayfurther comprise mechanical displacement stages for moving relativelythe object and the chromatic lens.

The mechanical displacement stages may comprise translation platesand/or rotation plates.

BRIEF DESCRIPTION OF THE DRAWINGS

The methods according to embodiments of the present invention may bebetter understood with reference to the drawings, which are given forillustrative purposes only and are not meant to be limiting. Otheraspects, goals and advantages of the invention shall be apparent fromthe descriptions given hereunder.

FIG. 1 illustrates a first mode of realization of confocal chromaticdevice for carrying out the method of the invention;

FIG. 2 illustrates a second mode of realization of confocal chromaticdevice for carrying out the method of the invention;

FIG. 3 illustrates a first mode of realization of measurement head witha magnifying lens;

FIG. 4 illustrates a second mode of realization of measurement head witha magnifying lens;

FIG. 5 illustrates a third mode of realization of measurement head witha magnifying lens;

FIG. 6 illustrates a fourth mode of realization of measurement head witha magnifying lens;

FIG. 7 illustrates a mode of realization of measurement head providingmeasurement points arranged in lines; and

FIG. 8 illustrates a flow chart of the method of the invention.

DETAILED DESCRIPTION

It is well understood that the embodiments described hereinafter are inno way limitative. Variants of the invention can in particular beenvisaged comprising only a selection of the features described below inisolation from the other described features, if this selection offeatures is sufficient to confer a technical advantage or todifferentiate the invention with respect to the state of the prior art.This selection comprises at least one preferred functional featurewithout structural details, or with only one part of the structuraldetails if this part alone is sufficient to confer a technical advantageor to differentiate the invention with respect to the state of the priorart.

In particular, all the described variants and embodiments can becombined if there is no objection to this combination from a technicalpoint of view.

In the figures, the elements common to several figures retain the samereferences.

With reference to FIG. 1 and FIG. 2, we will describe a confocalchromatic device for carrying out the method of the invention. FIG. 1and FIG. 2 illustrates several variant of implementation of somesubparts of the device, which may of course be combined.

The confocal chromatic device comprises a measurement head 12 with achromatic lens 13. Such lens is designed according to well-knowntechniques so as to provide a strong chromatic aberration, allowingdifferent optical wavelengths crossing the lens to be focused atdifferent axial distances (that is distances along the optical axis ofthe lens, or along the Z axis as shown in FIG. 1 and FIG. 2).

Of course, the chromatic lens 13 may comprise a single lens asillustrated in FIG. 1 and FIG. 2, or an arrangement of severalindividual lenses forming a chromatic lens assembly according towell-known techniques.

The confocal chromatic device further comprises several opticalmeasurement channels 24.

Each optical measurement channel 24 comprises a collection optical fiber17 for conveying the light to and from the measurement head 12 and thechromatic lens 13. In the mode of realization presented, thesecollection optical fibers 17 comprise multimode fibers arranged as abundle. The collection fibers 17 have an end 14 positioned in themeasurement head 12, which constitutes a collection aperture 14 of theconfocal detection set-up. These collection apertures 14 are located ina collection plane (corresponding to an X-Y plane in FIG. 1 and FIG. 2)relative to the chromatic lens 13.

Each optical measurement channel 24 allows doing measurements on ameasurement point 15 located in an object plane (corresponding to an X-Yplane) which is a conjugate plane of the collection plane for thechromatic lens 13. So, the measurement points 15 correspond to images ofthe collection apertures 14 by the chromatic lens 13, or, more preciselyand because of the chromatic dispersion, projections of the images ofthe collection apertures 14 for the various wavelengths in the objectplane. So the spatial repartition of the measurement points 15 in theobject plane is determined by the spatial arrangement of the collectionapertures 14 in the collection plane.

The optical measurement channels 24 are illuminated by a broadband lightsource 19. In the modes of realization presented, that light source 19may comprise a thermal source (halogen for instance) or a LED sourcegenerating light with wavelengths within for instance a range of 400-700nm (visible range).

In the mode of realization illustrated in FIG. 1, the light of the lightsource 19 is injected in optical fibers and conveyed through opticalcouplers 18 to the collection apertures 14. The couplers 18 may comprisefiber couplers, or couplers made with other technologies such as forinstance planar optics waveguides. They may be done with distinctcomponents for each optical measurement channel 24, or, in particularwhen using planar waveguide technologies, with components comprisingseveral couplers 18 for several measurement channels 24.

In the mode of realization illustrated in FIG. 2, the light of the lightsource is conveyed by illumination optical fibers 27 to the measurementhead 12. The measurement head 12 comprises a beam splitter 26 such as abeam splitter cube which directs the light issued from the illuminationoptical fibers 27 through their illumination aperture 28 (their end)towards the chromatic lens 13, and which allows coupling the lightreflected back by the object 10 to the collection apertures 14 of thecollection optical fibers 17. Two collimating lenses 29 are respectivelyarranged in front of the illumination apertures 28 and the collectionapertures 14 to ensure that the beams crossing the beam splitter 26 areessentially collimated. Of course the chromatic lens 13 is arrangedaccordingly.

The illumination apertures 28 and the collection apertures 14 arespatially arranged so as to form respectively pairs of conjugate pointswith a measurement point 15. For that, two similar collimating lenses 29are used and a same spatial repartition is done for the illuminationapertures 28 and the collection apertures 14.

The light of the light source 19 is focuses by the chromatic lens 13 sothat different wavelengths are focused at different axial positions onthe measurement points 15, thus defining a measurement range.

The light reflected at the measurement points 15 by an object ofinterest 10 positioned in the measurement range is coupled back in thecollection apertures 14. Thanks to the confocal arrangement of theset-up, only the light actually focused on an interface of the object 10is coupled back in the collection apertures 14, and the light reflectedby the object 10 out-of-focus is not coupled back. In addition, thanksto the chromatic dispersion of the chromatic lens 13:

-   -   The light focused on an interface (or a surface) of the object        10 correspond essentially to a single wavelength or group of        wavelength for which the focal length of the chromatic lens 13        corresponds to the axial optical distance to that interface        along the optical axis of the lens (corresponding to the Z        axis). So by analyzing the intensity spectrum of the reflected        light, the axial distance to the interfaces may be measured.        That measurement mode, which corresponds to a classical use of        the chromatic confocal technique, may be called profilometry        mode or 3D detection mode;    -   The light collected after reflection on an interface (or a        surface) of an object 10 located anywhere within the measurement        range does not include any significant defocused light but only        light focuses on that interface or surface. So it provides an        intensity information with a lateral resolution in the object        plane (X-Y) corresponding to the spot size at focus. And such        lateral resolution is achieved for interfaces or surfaces        located within the whole measuring range. So, by analyzing the        total intensity of the reflected light, the set-up allows        imaging interfaces or surfaces of the object 10 with a high        lateral resolution over an extended depth of focus. This        measurement mode has thus the advantage of allowing intensity        imaging of surfaces of structures 11 of a significant height (as        shown in FIG. 1 or FIG. 2) with an optimal lateral resolution in        a 2D (bidimensional) detection mode.

According to some modes of realization, the device of the inventioncomprises only optical measurement channels 24 with an intensitydetector 20 for measuring a total intensity of the collected light. Inthat case the device of the invention is devoted to fast 2D inspection(intensity imaging) with an extended depth of focus.

According to some modes of realization, the device of the inventioncomprises optical measurement channels 24 with (or coupled with) anintensity detector 20 and/or a spectral detector 21 for respectivelyacquiring data in 2D detection mode (intensity imaging) and/or 3Ddetection mode (profilometry).

In both cases, the light coupled back in the collection apertures 14 istransferred to these intensity detectors 20 and/or spectral detectors 21by the collection optical fibers 17 and, in the mode of realization ofFIG. 1, by the couplers 18.

Several arrangements of intensity detectors 20 and spectral detectors 21within or in relation with the optical measurement channels 24 arepossible. The device of the invention may notably comprise:

-   -   Optical measurement channels 24 which comprise only an intensity        detector 20 or a spectral detector 21. In that case, these        optical measurement channels 24 are dedicated to an intensity        (2D) measurement or an axial distance (3D) measurement at the        corresponding measurement point 15;    -   Optical measurement channels 24 which comprise an intensity        detector 20 and a spectral detector 21. These optical        measurement channels 24 further comprise a branching element 23        such as a coupler 23 or a switch 23 as illustrated in FIG. 1,        for directing the light coupled back in the collection apertures        14 simultaneously or sequentially towards the intensity detector        20 and the spectral detector 21. In that case, these optical        measurement channels 24 allow doing intensity measurements (2D)        and axial distance measurements (3D) at the corresponding        measurement point 15;    -   An optical multiplexer 25 with for instance an array of optical        switches 25 as illustrated in FIG. 2 operating as in        interconnection array and allowing interconnecting a plurality        of optical measurement channels 24 with a plurality of intensity        detectors 20 and/or spectral detectors 21 in a reconfigurable        way. In that case, these optical measurement channels 24 may be        configured on demand for doing an intensity (2D) measurement        and/or an axial distance (3D) measurement at the corresponding        measurement point 15.

The spectral detectors 21 as illustrated in the modes of realization ofFIG. 1 and FIG. 2 comprise:

-   -   An entrance pupil, corresponding for instance to an end of a        collection optical fiber 17, and a first lens for collimating        the incoming light issued from the entrance pupil;    -   A dispersing element such as a diffraction array or a grating        for dispersing angularly the different wavelengths of the        incoming light;    -   A second lens and a linear detector such as a line CDD for        re-imaging the dispersed light so that different wavelengths are        focused on different pixels of the sensor. The intensity        spectrum of the light is obtained by collecting the information        on the pixels of the sensor. An interface of the object 10        present in the measurement range gives rise to a peak in the        intensity spectrum around the wavelength focused at the        corresponding axial position. So the intensity spectrum is        analyzed to obtain an axial distance information, or the        position of the interfaces or the surface of the object 10        within the measurement range.

The spectral detectors 21 of the different measurement channels 24 maybe completely distinct, or, as illustrated in FIG. 2, they may sharesome elements such as the detector. For instance, several spectraldetectors 21 may share a same line or matrix sensor, the information ofeach spectral detector 21 being collected on a separate set of pixels ofthe shared detector. In the same way, several spectral detectors 21 mayshare a same dispersing element.

The intensity detectors 20 comprise point detectors such as photodiodeswhich measure the whole intensity of the light over the full spectrum.

The intensity detectors 20 of the different measurement channels 24 maybe distinct (using for instance individual photodiodes), or, asillustrated in FIG. 2, they may share some elements such as thedetector. For instance, several intensity detectors 20 may share a samephotodiode array, or a same line or matrix sensor (CCD or CMOS), theinformation of each intensity detectors 20 being collected on a separatepixel, set of pixel or photodiode.

In a variant of the mode of realization of FIG. 2, the collectionapertures 14 may be arranged directly at the level of the intensitydetectors 20 or the spectral detectors 21. In that case the measurementchannels 24 comprise no collection optical fibers 17 and of course nooptical multiplexer 25. For instance, the device of the invention maycomprise:

-   -   Intensity detectors 20 positioned with the sensing element or        sensing surface of their detector located in the collection        plane with the collection apertures 14, which is a conjugate        focal plane of the object plane with the measurement points 15.        The collection apertures 14 are then materialized directly by        the limited size of the sensing element (for instance when using        a photodiode) or by the limited size of the pixels when using        for instance a line or matrix CCD;    -   Intensity detectors 20 positioned with the sensing element or        sensing surface of their detector located behind a pinhole mask        or an entrance slit materializing the collection apertures 14        and positioned in the collection plane. An entrance slit may be        used to materialize a series of collection apertures 14 arranged        in line, facing for instance a line or matrix sensor shared        between several intensity detectors 20;    -   Spectral detectors 21 positioned with their entrance pupil        corresponding to the collection apertures 14 positioned in the        collection plane. These entrance pupils may be shaped as a        pinhole. They may also correspond to an entrance slit        materializing the entrance pupils of a series of spectral        detectors 21 sharing for instance a same dispersing element and        a matrix detector.

The device of the invention further comprises a computer or amicrocontroller 22 for control and data processing.

For allowing inspection of an object 10 such as a wafer, the device ofthe invention further comprises a holder for holding the object 10 (forinstance a wafer chuck) and a mechanical displacement stage 16 formoving relatively the measurement head 12 and the object 10. In the modeof realization presented, the mechanical displacement stage 16 maycomprise translation plates for linear displacements along the X, Y, andZ axis, and a rotation stage for rotating the object 10 (the wafer) inthe X-Y plane.

Of course the measurement head 12 may be distinct from the parts of thedevice holding the light source 19 and the detectors 20, 21, or thewhole system, including the measurement head 12, may be done as a singleassembly.

With reference to FIG. 3-FIG. 6, we will now describe some modes ofrealization of device of the invention allowing adjusting or varying thespatial separation of the measurement points 15 without mechanicallymoving the collection apertures 14.

Such mode of realization may be advantageous for instance for inspectingan object 10 with periodic structures 11. By adjusting the spatialseparation of the measurement points 15 to matches the period of thestructures 11, parallel 2D and/or 3D inspection of these structures 11at optimal speed may be performed.

According to these mode of realization, the measurement head 12 furthercomprise a magnifying lens 31 or a magnifying lens assembly 31 insertedbetween the collection apertures 14 and the chromatic lens 13. Themagnifying lens 31 is preferably an achromatic lens arrangement.

FIG. 3 illustrates a mode of realization in which the magnifying lens 31is arranged so as to image the collection apertures 14 in anintermediate conjugate focal plane 32 along the optical axis 35 with afirst magnification factor G″. If the collection apertures 14 areseparated by a distance d in the plane perpendicular to the optical axis35, their image 33 by the magnifying lens 31 is separated by a distanced″=G″d. The chromatic lens 13 is arranged so that the intermediateconjugate focal plane 32 is also a conjugate focal plane of the objectplane with the measurement points 15. So, by assuming a secondmagnification factor G′ for the chromatic lens 13 between theintermediate conjugate focal plane 32 and the plane of the measurementpoints 15, we obtain measurement points 15 separated by a distanced′=Gd, where G=G′G″ is the magnification factor G corresponding to theglobal magnification factor of the combination of magnifying lens 31 andchromatic lens 13. Of course, in all modes of realization presented, themagnification factor G may correspond to a magnification, a reduction,or a unity magnification.

It is to be noted that the lateral size of the measurement points 15,corresponding to the lateral resolution of these measurement points 15,is also changed by the magnification factor, but the ratio between theseparation distance d′ and the lateral resolution at the measurementpoints 15 is preserved, which is the most important for the quality ofthe sampling.

FIG. 4 illustrates a mode of realization in which the magnifying lens 31is arranged so that the collection apertures 14 are located in itsentrance focal plane. In that case, the intermediate conjugate focalplane is at infinity and the magnification factor G is determined by theratio of the focal lengths of the magnifying lens 31 and the chromaticlens 13. Of course the chromatic lens 13 is arranged to operate in suchconfiguration.

FIG. 5 and FIG. 6 illustrate modes of realization of measurement head 12with a magnifying lens 31 which are compatibles with the presence of abulk beam splitter 26 as described in the modes of realizations ofdevice illustrated in FIG. 2. Of course, these modes of realization mayalso be used with the modes of realization of device illustrated in FIG.1, without beam splitter 26 but using a collimating lens 29 arranged soas to have the collection apertures 14 in its focal plane.

In case of use of a beam splitter 26, the magnifying lens 31 is placedbetween the beam splitter 26 and the chromatic lens 13, so as to have asame magnification factor G applied to the collection apertures 14 andthe illumination apertures 28.

In the mode of realization of FIG. 5, the magnifying lens 31 is arranged(in combination with the collimating lens 29) so as to image thecollection apertures 14 in an intermediate conjugate focal plane 32 witha first magnification factor G″. In that case, the first magnificationfactor G″ it determined by the ratio of the focal lengths of themagnifying lens 31 and the collimating lens 29. As previously, byassuming a second magnification factor G′ for the chromatic lens 13between the intermediate conjugate focal plane 32 and the plane of themeasurement points 15, the (global) magnification factor G for thecombination of magnifying lens 31 and chromatic lens 13 corresponds toG=G′G″.

It is to be noted that, without beam splitter 26, the mode ofrealization of FIG. 5 is may be similar to the mode of realization ofFIG. 4 if the collimating lens 29 is part of the magnifying lensassembly 31.

In the mode of realization of FIG. 6, the magnifying lens 31 comprisesis an afocal lens arrangement, with for instance two lenses having theirintermediate focal planes superposed. In that case, the intermediateconjugate focal plane (between the magnifying lens 31 and the chromaticlens 13) is at infinity. The magnification factor G may be determined asbeing the product G=G′G″ of:

-   -   a first magnification factor G′ corresponding to the ratio of        the focal lengths of the chromatic lens 13 and the collimating        lens 29; and    -   a second magnification factor G″ corresponding to the ratio of        the respective focal lengths of the lenses of the afocal lens        pair comprised in the magnifying lens system 31.

It is to be noted that in the mode of realization of FIG. 6, themagnifying lens 31 has an infinite effective focal length, or in otherwords entrance and exit conjugate focal planes placed at infinity. Thatconfiguration has the advantage that the accuracy of the positioning ofthe magnifying lens 31 along the optical axis 35 is not critical for theperformance.

As previously explained, a purpose of the magnifying lens 31 is toprovide a capability to vary the magnification factor G of the opticalset-up, either continuously or within a discrete set of values.

Several practical implementations are possible.

According to some modes of realization, the magnifying lens 31 comprisesa zoom arrangement for varying continuously the magnification factor Gover a range.

For instance, in the mode of realization of FIG. 6, the magnifying lens31 may comprise an afocal zoom arrangement. According to a well-knownconfiguration, such afocal arrangement may comprise two converginglenses of equal focal length, and a diverging lens with an absolutefocal length less than half that of the converging lenses placed betweenthe converging lenses. Such arrangement allows varying the magnificationby moving the diverging lens and one of the converging lenses along theoptical axis 35 in a particular non-linear relationship.

According to some modes of realization, the measurement head 12comprises a mechanical mount 34 to change the magnifying lens 31.

The measurement head 12 may comprise for instance a turret 34 or alinear stage 34 holding several magnifying lens 31 and allowing tochange the magnifying lens 31 inserted between the collection apertures14 and the chromatic lens 13 by a translational or rotational movement.In that case, the different magnifying lenses 31 are arranged so that,once in place, the plane with the collection apertures 14 is conjugateof the object plane with the measurement points 15 by the whole opticalsystem, comprising the magnifying lens 31 and the chromatic lens 13. Ifat least one of the conjugate focal planes of the magnifying lens 31(that is the entrance plane towards the collection apertures 14 and/orthe intermediate conjugate focal plane 32) is at a finite distance,which is the case for the modes of realization of FIG. 3, FIG. 4 or FIG.5, the different magnifying lenses need to be positioned accuratelyalong the optical axis 35. If both conjugate planes of the magnifyinglens 31 are at infinite distance, which is the case for the mode ofrealization of FIG. 6, the requirements in terms of positioning alongthe optical axis 35 are relaxed.

The measurement head 12 may also comprise a turret or a linear stageholding several chromatic lenses 13 to be used with one fixed magnifyinglens 31 or several interchangeable magnifying lenses 31.

The modes of realization of FIG. 1-FIG. 6 show devices with a fewmeasurement channels 24 for sake of clarity. Of course, in practice adevice of the invention may comprise much more measurement channels 24,in the order of hundred or more.

The spatial repartition of the collection apertures 14 in themeasurement head 12 and the repartition of the intensity detectors 20and the spectral detectors 21 among the optical measurement channels 24may be of any kind, depending on the applications.

With reference to FIG. 7, we will now describe a mode of realization ofdevice optimized for allowing high-speed inspection a surface of anobject such as a wafer 10 with structures 11 such as bumps ormicro-bumps 11.

The optical measurement channels 24 are provided with collection fibers17 whose end forming the collection apertures 14 are arranged in twoparallel rows positioned in a mounting piece 43 (for instance withgrooved elements for accurately positioning the fiber ends).

A first row 41 comprises collection fibers 17 of measurement channels 24connected to intensity detectors 20.

A second row 42 comprises collection fibers 17 of measurement channels24 connected to spectral detectors 21.

The first row 41 and the second row 42 may have a same number ofcollection apertures 14 as illustrated in FIG. 4, or a different number,possibly with a different spacing.

The second row 42 may even have a single collection aperture 14connected to a single spectral detector 21.

Of course, the spatial repartition of the measurement points 15 may beadjusted using a magnifying lens 31 as described in relation with FIG.3-FIG. 6.

The main purpose of that specific arrangement, as it will be describedlater, is to provide a device which allows acquiring intensityinformation prior to axial distance information in a same scan.

Of course, other repartitions are possible. In particular, thecollection apertures 14 may be arranged in one row 41. And thesecollection apertures 14 may be optically connected to:

-   -   Only intensity detectors 20; or    -   Intensity detectors 20 or, for one or several collection        apertures located at the center of the row 41, spectral        detectors 21.

With reference to FIG. 8, we will now describe a method for inspecting asurface of an object in 2D and 3D modes.

Generally speaking, the method of the invention comprises steps of:

-   -   Acquiring an intensity information with several measurement        channels 24 on several measurement points 15 at the surface of        the object 10 (step 51);    -   Locating points of interests for axial distances measurements        using said intensity information and possibly intensity        information and/or axial distance information acquired during        preceding steps (step 52);    -   Positioning collection apertures 14 of at least one measurement        channel 24 with a spectral detector 21 over a point of interest        (step 53);    -   Acquiring at least one axial distance information (step 54);    -   repeating the process over the surface of the object 10 and        computing the results (step 55).

The computation may comprise for instance at least one of the following:Building a height map, building an intensity map, locating structures inthe X-Y plane, comparing height or in-plane dimensions of the structureswith expected values, issuing pass/fail data.

Optionally, the method may comprise a step of adjusting the spatialrepartition of the collection apertures 14 using a magnifying lens 31(step 50).

That adjustment may be done using a-priori knowledge on the object, orusing intensity information and/or axial distance information previouslyobtained. It may be done once at the beginning of the measurements orseveral times during the measurement process.

By using the set-up of the invention described in relation with FIG. 7,the method of the invention allows in particular doing a very high-speedinspection of a surface of a wafer 10 with structures 11 such as bumpsor micro-bumps 11 arranged in a periodic fashion.

In a first step, the measurement head 12 and the wafer 10 are arrangedso that the rows 41, 42 of collection apertures 14 are aligned with thestructures 11. Optionally the magnification is adjusted with themagnifying lens 31 so that the distance between the measurement points15 matches the spacing of the structures (with for instance onemeasurement point 15 on the top of the structure and one measurementpoint between two structures as illustrated on FIG. 7).

Then the measurement head is moved in a direction of displacement 44preferably perpendicular to the rows 41, 42 of collection fibers. Foreach displacement step:

-   -   An intensity information is acquired with the collection        apertures 14 of the first row 41. It is combined with the        previously acquired intensity information to build an intensity        map;    -   The intensity map is processed to locate the newly appearing        structures 11 along in the X-Y plane. The next points of        interest for axial distances measurements, corresponding for        instance to summits of structures 11 are computed accordingly;    -   If measurement points 15 corresponding to the collection        apertures 14 of the second row 42 are positioned on previously        identified points of interest, corresponding axial distance        information is acquired. Newly acquired axial information is        then combined with the previously acquired axial information to        build a height map.

The process is repeated over the whole area of interest of the wafer andthe data is computed for providing for instance at least one of thefollowing: a height map, an intensity map, location of structures in theX-Y plane, comparison of height or in-plane dimensions of the structureswith expected values, pass/fail data.

As previously, the magnification may be adjusted with the magnifyinglens 31 using a-priori knowledge on the object or intensity informationand/or axial distance information previously obtained once at thebeginning of the measurements or several times during the measurementprocess between displacement steps.

The devices and the methods of the invention may advantageously be usesfor several kind of applications. It may be used for instance forinspecting:

-   -   an object 10 such as a wafer;    -   an object 10 such as a wafer on carrier or glass carrier, or        wafer elements such as dies on a carrier;    -   an object 10 such as a wafer on frame, or wafer elements such as        dies on a frame;

In particular, for the inspection of an object 10 made of severalcompound elements such as dies on carrier or frame, the method of theinvention may comprise steps of:

-   -   performing inspection steps as previously described with a high        magnification leading to a coarse spatial resolution (and high        speed) for locating on the surface of the carrier or frame, in        the X-Y plane and possibly also in height Z, the compound        elements (or dies); and    -   performing inspection steps as previously described with a low        magnification leading to fine spatial resolution for inspecting        at least some of the compound elements (or dies), looking for        instance on solder bumps on these compounds elements.

While this invention has been described in conjunction with a number ofembodiments, it is evident that many alternatives, modifications andvariations would be or are apparent to those of ordinary skill in theapplicable arts. Accordingly, it is intended to embrace all suchalternatives, modifications, equivalents and variations that are withinthe spirit and scope of this invention.

The invention claimed is:
 1. A method for inspecting the surface of anobject comprising tridimensional structures, using a confocal chromaticdevice with a plurality of optical measurement channels and a chromaticlens with a strong chromatic aberration allowing different opticalwavelengths of a broadband light source to be respectively focused atdifferent axial distances, said different axial distances defining achromatic measurement range, the method comprising the steps of:illuminating a plurality of measurement points on the surface of theobject from the broadband light source through said chromatic lens, saidmeasurement points being conjugate points, with respect to saidchromatic lens, of collection apertures; measuring a total intensityover the full spectrum of the light collected through said collectionapertures on said illuminated measurement points by at least some ofsaid optical measurement channels in a confocal configuration forobtaining an intensity information of the light focused on the surfaceof said object at the measurement points, so as to provide an intensity2D image information of the surface and produce an image of the surfaceof said object with a high lateral resolution over a depth of focusextending over the chromatic measurement range; locating a structure onthe surface of said object using said intensity information to identifya measurement point of interest relative to said located structure; andobtaining an axial distance information within said chromaticmeasurement range at said point of interest by measuring a spectralinformation of the light collected through said collection apertures byat least one of said optical measurement channels.
 2. The method ofclaim 1, further comprising at least one of the following steps:identifying a measurement point of interest relative to a structure;identifying a measurement point of interest relative to a structurecorresponding to a summit of said structure.
 3. The method of claim 2,further comprising a step of deducing a height information of saidstructure.
 4. The method of claim 1, further comprising a step of movingrelatively the object and the chromatic lens to position an opticalmeasurement channel so as to obtain an axial distance information on apreviously identified measurement point of interest.
 5. The method ofclaim 1, comprising a step of moving relatively the object and thechromatic lens along a pre-defined scan trajectory, and for a scanposition: obtaining an intensity information; and/or obtaining an axialdistance information on a measurement point of interest previouslyidentified.
 6. The method of claim 1, further comprising at least one ofthe following steps: changing a spacing of measurement points; changinga spacing of measurement points by changing a scaling factor between aspatial repartition of collection apertures of the optical measurementchannels and the measurement points using a magnifying lens.
 7. Themethod of claim 6, further comprising at least one of the followingsteps: adjusting a spacing of measurement points taking into account aspatial repartition of structures on the object; adjusting a spacing ofmeasurement points so as to substantially match a spacing of structureson the object.
 8. The method of claim 6, further comprising at least oneof the following steps: obtaining an information on the spacing ofstructures using a-priori knowledge on the object; obtaining aninformation on the spacing of structures using intensity informationand/or axial distance information previously obtained.
 9. The method ofclaim 6, further comprising steps of: obtaining an intensity informationand/or an axial distance information at a plurality of measurementpoints with a first spacing of said measurement points, for locatingsub-elements on the surface of the object; obtaining an intensityinformation and/or an axial distance information at a plurality ofmeasurement points with a second spacing of said measurement pointsfiner than the first spacing on a sub-element.
 10. The method of claim1, further comprising steps of: building an intensity image by combiningintensity information obtained in a region of interest of the object;and/or building a height map by combining axial distance informationobtained in a region of interest of the object.
 11. The method of claim1, further comprising a step of comparing obtained axial distanceinformation with reference value(s).
 12. The method of claim 1, which isimplemented for inspecting tridimensional structures of at least one ofthe following type: bumps, micro-bumps, solder bumps, copper pillars,copper nails, Re-Distribution Layers (RDL), metal patterns.